In many installations, inverters are used to drive the motor and the load of the installations. The inverters are provided with a protector circuit that functions, while the inverters are working, for protection against anomalies such as overcurrent protection, overload protection, over-voltage protection, and under-voltage protection to make the installations stop operating when any anomaly is detected. For managing the working situations of the installations, it is necessary for the installation manager to know the stop time of the inverter and the installations.
For monitoring the time of the stop due to the operation of the protector circuit in the inverter, the clock function of a programmable controller ranking high in the installation management control apparatuses is used generally. Although the stop time monitoring using the programmable controller ranking high does not pose any problem on the accuracy thereof, the monitoring costs soars and it is not possible to display the stop time on the display device of the inverter.
Unexamined Laid Open Japanese Patent Application Hei. 6 (1994)-14558 discloses an inverter that facilitates closely examining the causes of anomalies. While the inverter is working, data such as the driving status, the output frequency, the output current, and output voltage of the inverter are written in at a certain time interval. The disclosed inverter is provided with a memory that stores the above described data for a predetermined period of time from the past to the present and the data writing into the memory is stopped at the instance an anomaly is detected. After an anomaly is caused, the stored contents of the memory is displayed on a display device disposed outside the inverter or a display device on the inverter so that the process, through which the anomaly is caused, and the cause of the anomaly may be examined closely. However, the costs of the inverter increases, since it is necessary for the inverter to equip the memory for storing a lot of data relevant to the anomaly caused and a processor that controls the data storage. For simply obtaining the period from the time at which the inverter stopped, the operations conducted by the disclosed inverter for obtaining the necessary data may be complicated too much to obtain the necessary data.
The inverter is controlled with a microcomputer driven by a quartz oscillator and such a clock and capable of measuring the time internally. In utilizing the clock in the microcomputer, an inexpensive oscillator is used generally. In other words, a less precise clock is employed generally. Therefore, a larger error is caused as compared with the quartz clock as the counted time is longer. Since a battery is generally not disposed in the inverter, it will be necessary, once the power supply is interrupted, to reset the time, even if an expensive and precise oscillator is employed, causing a heavy burden on the user, who is the manager of the installations.
In view of the foregoing, it would be desirable to provide an inverter that facilitates informing the installation manager of the period of time from the instance, thereat the inverter is stopped due to an anomaly and such a cause. It would be also desirable to provide an inverter, which the user such as an installation manager can operate easily, with low manufacturing costs. It would be further desirable to provide an inverter that facilitates measuring a period of time with reasonable precision.